Articulating disc implant

ABSTRACT

An intervertebral disc implant for use in the spine including a first part and a second part, wherein the first part and second part are configured as a joint prosthesis for the spine. The first part includes one of a concave or convex articulating surface and the second part includes one of the other concave or convex articulating surface. One of the concave or convex articulating surfaces is preferably elliptically shaped in at least one direction and does not match and is different than one of the other concave or convex articulating surfaces.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/513,462 filed Oct. 14, 2014, which is a continuation of U.S.application Ser. No. 12/995,723, filed Dec. 2, 2010, which is a NationalStage of International Application No. PCT/US2009/046442, filed Jun. 5,2009, which claims the benefit of U.S. Provisional Application No.61/059,024, filed Jun. 5, 2008, the disclosures of which areincorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

This invention relates to an implant, more specifically a jointprosthesis, more specifically an intervertebral implant, and even morespecifically a joint prosthesis to replace a spinal disc.

BACKGROUND OF THE INVENTION

Due to general wear and tear, spinal discs can become damaged ordislocated giving rise to a problem commonly referred to as a “slippeddisc”. Intervertebral spinal discs lie between adjacent vertebrae in thespine. Each disc forms a cartilaginous joint allowing slight movement ofthe vertebrae and acting as a ligament to hold the vertebrae together.In the past, damaged discs were treated by removing the disc and packingthe space with bone chips to promote fusion of the adjacent vertebrae.However, this method resulted in a loss of mobility in the patient'slower back. Another solution for treating damaged discs is to replacethe damaged disc with a prosthetic disc implant. However, currentprosthetic disc implants do not replicate the ranges of motionundertaken by healthy spinal vertebrae. Thus, there is a need for aprosthetic disc implant that can more closely approximate and permit theranges of motion typically experienced by healthy spinal segments.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to an implant, more particularly ajoint prosthesis, more preferably an intervertebral implant or jointprosthesis to replace a spinal disk. The intervertebral implant orartificial disc replacement device may have particular application inthe cervical regions of the spine. In one embodiment, the intervertebralimplant includes a first part and a second part wherein the first andsecond parts may be configured as a joint prosthesis preferably for thespine, and where the first part is moveable, preferably in situ,relative to the second part.

The first part has a surface for contacting the end plates of a firstvertebrae and a first articulating surface, preferably having acontinuously curved surface with no flat regions. The second partincludes a surface for contacting the end plates of a second vertebraeand a second articulating surface, preferably having a continuouslycurved surface with no flat regions. The first articulating surface maycontact, bear on, and be movable relative to the second articulatingsurface. The first articulating surface may be concave, and the secondarticulating surface may be convex.

The first articulating surface preferably is a different shape than thesecond articulating surface so that the two shapes do not match or nest,and the surfaces preferably will not contact over a majority orsubstantial majority of their surfaces. The first articulating surfacepreferably is elliptical in shape (i.e., a flattened circle or oval),preferably partially oblate ellipsoid and has a radius of curvature thatvaries, i.e., changes, along its length, preferably in at least themedial-lateral direction, although it may have an elliptical surface inother directions with a varying radius of curvature. The secondarticulating surface preferably is partially spherical in shape, with aconstant radius of curvature that is the same in all directions. Theradius of curvature of the first articulating surface in themedial-lateral direction (e.g., a first direction), although it may varyand change over its length, preferably has a different value than theradius of curvature of the second articulating surface in the samemedial-lateral direction. The value of the first radius of curvature ofthe first articulating surface in the medial-lateral direction (althoughit may vary and change over its length), preferably may be greater thanthe value of the first radius of curvature of the second articulatingsurface in the same direction.

The second radius of curvature of the first articulating surface in theanterior-posterior direction (e.g., a second direction) may berelatively and substantially constant. The second radius of curvature ofthe first articulating surface in the anterior-posterior direction(e.g., the second direction) may have substantially the same value asthe second radius of curvature of the second articulating surface in thesame anterior-posterior direction. In this embodiment, where the radiusof curvature is not the same in one direction but the same in a seconddirection, the first articulating surface may generally have a line ofcontact, or limited area of contact with the second articulatingsurface. Alternatively, the value of the second radius of curvature ofthe first articulating surface may be different than the value of thesecond radius of curvature of the second articulating surface in thesame second direction. In this embodiment, where the radius of curvatureis not the same in all directions the first articulating surface mayhave generally a point of contact or a limited area of contact with thesecond articulating surface.

The first and second articulating surfaces may be structured andarranged such that the first articulating surface undergoes a rollingmotion relative to the second articulating surface in at least onedirection, and preferably rolls on and along the articulating surface ofthe second part in at least one direction. During the rolling motion,the first part may axially rotate and roll on and along the secondcurved articulating surface. In this case, the instantaneous point ofrotation of the first part moves on and along the second articulatingsurface in at least one direction.

The radius of curvature of the first and second articulating surfacesmay be between about 1 mm and about 100 mm, and more preferably betweenabout 1 mm and 30 mm. The first and second articulating surface may havean arc length between about 1.5 mm to about 30 mm.

The first articulating surface may have a depth, which is measured fromthe outer edge or the base point of the first articulating surface wherethe indentation or trough starts to the apex of the first articulatingsurface. The depth of the first articulating surface may range fromabout 0.5 mm to about 10 mm. The second articulating surface may have aheight, which is measured from the outer edge or the base of the secondarticulating surface to the apex of the second articulating surface. Theheight of the second articulating surface may range from about 0.5 mm toabout 10 mm. Preferably the value of the height of the secondarticulating surface is more than the value of the depth of the firstarticulating surface.

The surface of the first part for contacting the end plates of the firstvertebrae preferably may be relatively flat and the surface of thesecond part for contacting the end plates of the second vertebrae may becurved. Alternatively, the surface for contacting the end plates of avertebra of the first part may be curved, and additionally the surfacefor contacting the end plates of a vertebra of the second part may berelatively flat. The first part and the second part may be made ofceramic or other materials, now known or hereafter discovered. The firstpart may comprise a multi-piece assembly and the second part also maycomprise a multi-piece assembly.

The first part and second part may be biased so that the apex of thefirst curved articulating surface tends to align with and contact theapex of the second curved articulating surface. That is the shape andgeometry of the trough (concave surface) enables the articulating orbearing surface of the implant to return to a state of minimum energy(apex to apex) after the muscles no longer exert any forces on thespinal segment. The implant will tend to move back to its naturalposition wherein the portion of the convex surface in contact with theconcave surface is located at the apex of the curvate concavearticulating surface after lateral or rotation movement.

Another embodiment of the intervertebral implant includes an upper partincluding an upper surface sized and configured to contact an end plateof the first vertebra and a lower part including a lower surface sizedand configured to contact an end plate of the second vertebra. Theintervertebral implant further includes a convex articulating surfaceoperatively associated with one of the upper and lower parts. Theintervertebral implant also includes a concave articulating surfaceoperatively associated with the other one of the upper and lower partsso that the upper part is moveable with respect to the lower part. Theconvex articulating surface may be shaped as a partial sphere with aconstant radius of curvature that is the same in all directions. Theconcave articulating surface preferably may be shaped as a partialellipse or oval with a radius of curvature that changes over the lengthof the curved concave articulating surface in at least one direction.The convex and concave articulating surface may be continuously curvedwith no flat regions.

The convex articulating surface has a first radius of curvature in afirst direction and concave articulating surface has a first radius ofcurvature in the same first direction. The first radius of curvature ofthe convex articulating surface is preferably different than the firstradius of curvature of the concave articulating surface in the samefirst direction. The first direction preferably may be themedial-lateral direction and the concave articulating surface preferablymay be associated with the upper part.

The convex articulating surface may have a second radius of curvature ina second direction perpendicular to the first direction, preferably theanterior-posterior direction, and the concave articulating surface mayhave a second radius of curvature in the same second direction. Thesecond radius of curvature of the convex articulating surface may be thesame as the second radius of curvature of the concave articulatingsurface in the same second direction. Alternatively, the second radiusof curvature of the convex articulating surface may be different thanthe second radius of curvature of the concave articulating surface inthe same second direction. The convex articulating surface may have aradius of curvature in the anterior-posterior direction and themedial-lateral direction of about 1 mm to about 30 mm. The concavearticulating surface may have a first radius of curvature in theanterior-posterior direction of about 1 mm to about 100 mm, morepreferably about 1 mm to about 30 mm, and may have a second radius ofcurvature in the medial-lateral direction of about 1 mm to about 100 mm.

The convex articulating surface may have a constant radius of curvaturein its lateral-medial direction, and in its anterior-posteriordirection. Preferably, the concave articulating surface may have aradius of curvature that varies or changes along its surface in themedial-lateral direction, and preferably has a constant radius ofcurvature in its anterior-posterior direction. Alternatively, theconcave articulating surface may have a radius of curvature that variesor changes along its surface in the anterior-posterior direction.

The concave articulating surface may indent a depth from its base to itsapex, and the convex articulating surface may project a height from itsbase to its apex. The depth of the concave articulating surface may havea different value than the height of the convex articulating surface.The depth of the concave articulating surface preferably may have avalue less than the value of the height of the convex articulatingsurface. The depth of the concave articulating surface is preferablybetween about 1.5 mm and about 2 mm, and the height of the convexarticulating surface is preferably between about 2 mm and about 3 mm.

The convex articulating surface may have an arc length in theanterior-posterior direction and the medial-lateral direction of about1.5 mm to about 30 mm. The convex articulating surface may have an arclength in the medial-lateral direction and the anterior-posteriordirection of about 1.5 mm to about 30 mm.

The concave articulating surface may only contact the convexarticulating surface over a limited area, preferably an area comprisingless than 50% of the area of the concave articulating surface, morepreferably an area comprising less than 25% of the concave articulatingsurface, and more preferably an area comprising less than 10% of theconcave articulating surface. The convex articulating surface may formgenerally a line of contact or limited linear area of contact with theconcave articulating surface. Alternatively, the convex articulatingsurface may form generally a point of contact or circular area ofcontact with the concave articulating surface that preferably is lessthan the area of contact for the embodiment that has the line or limitedlinear area of contact.

In another embodiment the implant includes an upper part including anupper surface sized and configured to contact bone, an end plate of anupper vertebra, a lower part including a lower surface sized andconfigured to contact bone, a convex articulating surface operativelyassociated with one of the upper and lower parts, a concave articulatingsurface operatively associated with the other one of the upper and lowerparts so that the upper part is moveable with respect to the lower part,wherein one of the concave and convex articulating surfaces iselliptically shaped with a radius of curvature that varies in at leastone direction and the other of the concave and convex surfaces ispartially spherically shaped having a constant radius of curvature inthe same at least one direction. Preferably the other of the convex andconcave articulating surfaces is partially spherically shaped having aconstant radius of curvature that is the same in all directions.

The intervertebral implant may further include a first joint member, thefirst joint member including one of the convex and concave articulatingsurfaces. The first joint member may further include a first projectionor a first recess for engaging one of a first recess or a firstprojection formed in the lower part. The intervertebral implant mayfurther include a second joint member. The second joint member mayinclude the other one of the convex and concave articulating surfacesand one of a second projection and a second recess for engaging one of asecond recess and second projection formed in the upper part.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe preferred embodiments of the application, will be better understoodwhen read in conjunction with the appended drawings. For the purposes ofillustrating the preferred intervertebral implant and/or spineprosthesis of the present application, drawings of the preferredembodiments are shown. It should be understood, however, that theapplication is not limited to the precise arrangement, structures,features, embodiments, aspects, and instrumentalities shown, and thatthe arrangements, structures, features, embodiments, aspects andinstrumentalities shown may be used singularly or in combination withother arrangements, structures, features, aspects, embodiments andinstrumentalities. In the drawings:

FIG. 1 is a schematically representation of the spine in the cervicalregion and the movement of adjacent vertebrae.

FIG. 2 is an illustration of a path of motion of cervical vertebraeabout axis 1 of FIG. 1, which is projected onto the axial plane (theplane of axis 2 of FIG. 1).

FIG. 3 is a perspective view of an intervertebral implant of the presentinvention attached to adjacent vertebrae.

FIG. 4 is a perspective exploded view of the intervertebral implant ofFIG. 3.

FIG. 5 is a cross-sectional view of the intervertebral implant of FIG. 3in the anterior-posterior direction (sagittal plane).

FIG. 6 is a cross-sectional view of the intervertebral implant of FIG. 3in the medial-lateral direction (coronal plane).

FIG. 7 is a cross-sectional view of a different embodiment of anintervertebral implant in the anterior-posterior direction (sagittalplane).

FIG. 8 is a cross-sectional view of the intervertebral implant of FIG. 7in the medial-lateral direction (coronal plane).

FIG. 9 is a bottom view of the top part of FIG. 3.

FIG. 10 is a top view of the bottom part of FIG. 3.

FIG. 11 is an illustration of a path of motion of a vertebra about axis1 of FIG. 1 in the lumbar region of the spine.

FIG. 12 is a schematic illustration of the bottom part of anintervertebral implant at different locations along the path of FIG. 11.

FIG. 13 is bottom view of the top part (with a projection of the bottompart) of another embodiment of an intervertebral implant designed to beimplanted in the lumbar region of the spine.

FIG. 14 is an illustration of the intervertebral implant of FIG. 5 withtop part rotated in the anterior-posterior direction.

FIG. 15A is a cross-sectional illustration of the intervertebral implantof FIG. 6 with the top part centered in the medial-lateral direction.

FIG. 15B is a cross-sectional illustration of the intervertebral implantof FIG. 6 with the top part rotated in the medial-lateral direction.

FIG. 15C is a schematic illustration of the path taken by the contactpoint 200 in FIGS. 15A-B during rotation of the top part of theintervertebral implant 100 of FIGS. 3-6 in the medial-lateral direction.

DETAILED DESCRIPTION OF THE INVENTION

Certain terminology is used in the following description for convenienceonly and is not limiting. The words “right”, “left”, “lower”, “upper”,“bottom”, and “top” designate directions in the drawings to whichreference is made. The words “inwardly” and “outwardly” refer todirections toward and away from, respectively, the geometric center ofthe bone fixation element, instruments and designated parts thereof. Thewords, “anterior”, “posterior”, “superior”, “inferior”, “medial”,“lateral” and related words and/or phrases designate preferred positionsand orientations in the human body to which reference is made and arenot meant to be limiting. The terminology includes the above-listedwords, derivatives thereof and words of similar import.

FIG. 1 illustrates two adjacent cervical vertebrae, vertebra 101 andvertebra 201, which are located in the spine. Vertebrae 101 and 102typically have a spinal disc (not shown) positioned between them thatform a spinal segment that permits and facilitates movement of thevertebrae relative to one another. When a person moves their body,muscles exert pressure on vertebrae 101 and 201 causing them to move. Inthe cervical region when the vertebrae 101 and 102 move relative to eachother they essentially rotate about axis 1, axis 2, or both axis 1 and2. Referring to FIG. 1, axis 9 is the central axis of the spine andcorresponds to the intersection of the medial-lateral and theanterior-posterior planes. In the cervical region, rotational axis 1creates an angle X at the intersection of the axial plane, whichincludes axis 2. The value of angle X may be between about 10 degreesand about 60 degrees depending upon the location of the vertebrae in thecervical region of the spine. For example, axis 1 about which thecervical vertebrae C5 rotates about cervical vertebrae C6 is inclinedabout 45 degrees (X=45 degrees) with respect to the axial plane.

When a person bends their head forward, such as to look at their toes,or backwards, such as to look at the sky, the spine undergoes a motionknown as flexion and extension, respectively. When the cervical spineundergoes pure flexion and pure extension, vertebrae 101 and 201 rotateabout axis 2 and move in the sagittal plane.

When a person bends their head side to side, the cervical spineexperiences a motion known as lateral bending. To describe this motionwe need a fixed reference. In this case we chose the lower vertebrae ofa motion segment (composed of inferior vertebrae, superior vertebrae anddisc). Due to angle X that the cervical vertebrae makes with the axialplane, when a person bends their head from side to side, the spine willexperience a combined motion between axial rotation and lateral bending.This is, during lateral bending, axial rotation is induced in theadjacent vertebrate. Therefore if the head is moved laterally it willinduce axial rotation in the vertebrae. When cervical vertebrae 101 and201 undergo pure lateral bending, the vertebrae rotate about both axis 1and axis 2 to move in the coronal plane.

Finally, when a person twists their body or parts of their body, such asto turn their head left to right, the spine experiences a motion knownas axial rotation. Due to angle X that the cervical vertebrae make withthe axial plane, when a person twists their head, the spine experiencesa combined motion between axial rotation and lateral bending. Forexample, during axial rotation, lateral bending is induced in theadjacent vertebrae. When the head undergoes pure axial rotation,cervical vertebrae 101 and 201 move about axis 1 and axis 2 so that thehead will move in the axial plane as if rotating about reference axis 9.

FIG. 2 illustrates the motion experienced by vertebrae 101 and 201 fromFIG. 1 during lateral bending of the spine. As shown in FIG. 1, duringlateral bending, vertebrae 101 and 201 rotate about axis 1, creating apath of motion 5. When this path of motion 5 is projected onto axialplane 3, corresponding to the plane of axis 2, curved line 6 results.This curved line 6 corresponds to the path along which healthy vertebrae101 and 201 move relative to one another when projected onto the axialplane 3. The line or path 6 is in the shape of an ellipse with a varyingor changing radius of curvature.

FIG. 3 is an illustration of a prosthetic intervertebral implant 100positioned between vertebrae 101 and 201 from FIG. 1. Whileintervertebral implant 100 is described for use in the spine,intervertebral implant 100 may have other uses and may be used as aprosthesis for other joints, such as, for example, the shoulder, elbow,wrists, hip, knee, ankle, toes and fingers. Intervertebral implant 100preferably is for use in the cervical region of the spine. When used inthe cervical region, intervertebral implant 100 preferably allows forthe adjacent vertebrae (between which the implant is located) toexperience the following ranges of motion, as described above: (i) about±10 degrees during flexion and extension, (ii) about ±7 degrees duringlateral bending, and (iii) about ±7 degrees during axial rotation.Alternatively, intervertebral implant 100 may be designed for otherregions of the spine, such as, for example, the lumbar or thoracicregions. When the intervertebral implant 100 is designed for use in thelumbar region, the intervertebral implant 100 preferably allows for thespine to experience the following ranges of motion: (i) about ±10degrees during flexion and extension, (ii) about ±7 degrees duringlateral bending, and (iii) about ±10 degrees during axial rotation.Other ranges of motion may also be permitted by the implant.

FIGS. 3-10 and 13-15 illustrate exemplary embodiments of anintervertebral implant and more particularly a joint prosthesis forreplacing a spinal disc. In general, such embodiments relate to anintervertebral implant 100, 100′, 100″, by way of non-limiting example,an intervertebral implant 100, 100′, 100″ for replacement ofintervertebral discs or intervertebral fibro cartilage, which liesbetween adjacent vertebrae. While intervertebral implant 100, 100′, 100″will be described as and may generally be used in the spinal regions(e.g., lumbar, thoracic, cervical), those skilled in the art willappreciate that intervertebral implant 100, 100′, 100″ may be used inother parts of the body. The invention may have other applications anduses and should not be limited to the structure or use described andillustrated. Generally, the same reference numerals will be utilizedthroughout the application to describe similar or the same components ofeach of the different embodiments of the intervertebral implant and thedescriptions generally will focus on the specific features of theindividual embodiments that distinguish that particular embodiment fromthe others.

FIG. 4 illustrates one embodiment of an intervertebral implant accordingto the present invention. As shown in FIG. 4, intervertebral implant 100preferably includes a first part 10 and a second part 20. Intervertebralimplant 100 is designed so that in situ first part 10 moves relative tosecond part 20. First part 10 and second part 20 are configured to forma joint prosthesis to partially or fully replace a joint, such as, forexample, a disc between two adjacent vertebrae. In the embodiment shownand illustrated in FIGS. 4-6, intervertebral implant 100 is intended toreplace a spinal disc and preferably will permit the type and degree ofmotion generally experienced by healthy, adjacent cervical vertebrae.Implant 100 is designed particularly for application in the cervicalregions of the spine. First part 10 and second part 20 each have asurface for abutting against and preferably engaging the end plates ofadjacent vertebrae. First part 10 and second part 20 each also have anarticulating or bearing surface that preferably oppose, contact, bear onand move relative to each other.

While first part 10 and second part 20 are each illustrated as singlepiece assemblies, each of first part 10 and second part 20 may comprisea multi-piece assembly. Furthermore, as generally understood by one ofordinary skill in the art, first part 10 and second part 20 can be madefrom any number of biocompatible materials, including, but not limitedto ceramic, CoCr, PEEK, partially porous PEEK components, polymers,allograft bone, autograft bone, metals and alloys, and/or combinationsthereof now known or later discovered.

As shown in FIG. 3, first part 10 is preferably the top or upper part ofthe implant and engages the superior vertebrae. More particularly, firstpart 10 has a first surface 15 for contacting vertebra 101 that isrelatively flat. In alternative embodiments, first surface 15 may beslightly curved or substantially curved. First part 10 may furtherinclude one or more keels 17 preferably disposed in theanterior-posterior direction to secure the first part 10 to vertebra101. While in this embodiment keel 17 is illustrated as the means forsecuring first part 10 to vertebra 101, additional or alternativesecuring means or elements may be used, such as, for example, teeth,ridges, in-growth areas, or screws.

As shown in FIGS. 4 and 9, first part 10 also includes a firstarticulating surface 16, which is preferably a continuously curvedsurface with no flat portions. The first articulating surface 16 ispreferably concave and preferably has the three dimensional shape of apartial oblate ellipsoid. In alternative embodiments, first articulatingsurface 16 may be a partial sphere and/or parabolically shaped. Thepreferred shape of the indentation or trough of the concave articulatingsurface in the anterior-posterior direction (saggital plane) ispartially circular or spherical, having a constant radius of curvature.The preferred shape of the indentation or trough of the concavearticulating surface 16 of first part 10 in the medial-lateral direction(coronal plane) is elliptical (oval-shaped) and preferably correspondsto dashed line 6 from FIG. 2. That is, in the preferred shape of thefirst articulating surface 16, the radius of curvature varies, e.g.,changes, along the surface in the medial-lateral direction.

As shown in FIG. 4, second part 20 also includes a second articulatingsurface 26, which is preferably a continuously curved surface with noflat portions. The second articulating surface is preferably convex andhas a three dimensional shape of a partial sphere. The radius ofcurvature of the second articulating surface is preferably constant andthe same in all directions.

Accordingly, the first articulating surface 16 preferably does not matchor correspond to the second articulating surface 26 so that the firstcurved articulating surface does not contact the second articulatingsurface over its entire surface or the entire surface of the secondarticulating surface. That is, for the embodiment of FIGS. 4-6, as aresult of the partial elliptical or oval shape of the first articulatingsurface and the partial spherical shape of the second articulatingsurface, the two surfaces do not match, correspond or make contact overthe entire areas of or substantially the entire areas of either of thearticulating, bearing surfaces.

FIG. 5 illustrates a cross-sectional view of intervertebral implant 100in the anterior-posterior direction along the sagittal plane. Firstarticulating surface 16 preferably is indented and forms a trough in thehorizontal plane of first part 10 that starts along the edge or line1001. First articulating surface 16 has a first arc 12 in theanterior-posterior direction, measured from base point 1000 on theanterior side of edge 1001 to base point 1010 on the posterior side ofedge 1001. Base points 1000 and 1010 are the end points of firstarticulating surface 16 in the anterior-posterior direction, and betweenwhich the surface is preferably continuously curved with no flatportions. First arc 12 of first articulating surface 16 preferably has alength between about 3 mm to about 30 mm, more preferably about 8 mm toabout 15 mm, and more preferably about 10 mm to about 14 mm.

As shown in FIG. 5, first articulating surface 16 has a first radius ofcurvature 18 in the anterior-posterior direction, preferably betweenabout 1 mm and about 30 mm, more preferably between about 5 mm and about10 mm, and more preferably about 7.5 mm. Preferably, first radius ofcurvature 18 of first articulating surface 16 is substantially constantalong the first arc 12 forming a partially circular or spherical surfacein the anterior-posterior direction.

First part 10 further has a first depth 13 measured from base point1000, 1010, where the indentation or trough of the concave articulatingsurface 16 starts at edge 1001, to apex 1090 of first articulatingsurface 16. First depth 13 preferably has a value between about 0.5 mmto about 5 mm, more preferably about 1 mm to about 2 mm, more preferablyabout 1.8 mm.

FIG. 6 illustrates a cross-sectional view of intervertebral implant 100in the medial-lateral direction. First articulating surface 16 has asecond arc 32 in the medial-lateral direction, measured from base point1040 on the medial side of edge 1041 to base point 1050 on the lateralside of edge 1051. Base points 1040 and 1050 are the end points onopposite ends of first articulating surface 16 in the medial-lateraldirection, and between which the surface is preferably continuouslycurved with no flat portions. Second arc 32 of first articulatingsurface 16 preferably has a length between about 3 mm to about 30 mm,more preferably about 10 mm to about 20 mm, and more preferably about 10mm to about 15 mm.

As illustrated in FIG. 6, first articulating surface 16 preferably has asecond radius of curvature 38 in the medial-lateral direction. Thesecond radius of curvature 38 of first articulating surface 16preferably is between about 1 mm and about 100 mm. In this embodiment,second radius of curvature 38 varies or changes along the firstarticulating surface 16 in the medial-lateral direction. The secondradius of curvature 38 of the first articulating surface 16 along secondarc 32 preferably varies between about 1 mm and about 100 mm, morepreferably about 7.5 mm to about 50 mm, and more preferably about 7.5 mmto about 30 mm.

Furthermore, first part 10 further has a second depth 43 measured frombase point 1040, 1050, where the indentation or trough of the concavearticulating surface 16 starts at edge 1001, to apex 1090 of firstarticulating surface 16. Second depth 43 preferably has a value betweenabout 0.5 mm to about 5 mm, more preferably about 1 mm to about 2 mm,more preferably about 1.8 mm. Preferably, first depth 13 and seconddepth 43 are equal, and apex 1090 is located at the center of concavesurface 16. Alternatively, first depth 13 may be different than seconddepth 43, and the apex 1090, or the deepest part of indentation 16 maybe located off-center from the geometrical center of the indentation.

Referring to FIGS. 5-6, the value of first radius of curvature 18 offirst articulating surface 16 is constant while the value of secondradius of curvature 38 of first articulating surface 16 varies orchanges along the length of the surface in the medial-lateral direction.While the value of the radius of curvature of the first articulatingsurface 16 in the second medial-lateral direction may vary, the value offirst radius of curvature 18 of first articulating surface 16 may bedifferent than and preferably less than the value of second radius ofcurvature 38 of first articulating surface 16 as shown in FIG. 6.Alternatively, the value of first radius of curvature 18 of firstarticulating surface 16 may be greater than, or equal to, the value ofthe second radius of curvature 38 of first articulating surface 16.Preferably, the length of first arc 12 of first articulating surface 16is less than the value of second arc 32 of first articulating surface16. Alternatively, the value of first arc 12 of first articulatingsurface 16 may be greater than, or equal to the value of second arc 32of first articulating surface 16. Preferably, the value of first depth13 is equal to the value of second depth 43. Alternatively, the value offirst depth 13 may be greater than or less than the value of seconddepth 43.

As shown in FIGS. 3-4, second part 20 is preferably the bottom or lowerpart of the implant and engages the inferior vertebrae. Preferably,second part 20 has a first surface 25 for contacting vertebra 201 thatis relatively flat. Alternatively, first surface 25 may be slightlycurved or substantially curved. Second part 20 may further include oneor more keels 27 disposed in the anterior-posterior direction to securethe second part 20 to vertebra 201. While in this embodiment keel 27 isillustrated as the means for securing second part 20 to vertebra 201,alternate or additional securing means or elements now known or laterdiscovered may be used, such as, for example, teeth, ridges, bonein-growth areas, or screws.

As shown in FIG. 4, second part 20 includes a second articulatingsurface 26 that preferably projects outward from second part 20, isconvex and has the three dimensional shape of a partial sphere. FIG. 10shows a top view of the second part 20 of FIG. 3, and in particular theconvex second curved articulating surface 26, while FIGS. 4, 5, and 6show a perspective side view, an anterior-posterior cross-section(sagittal plane) and a medial-lateral cross section (coronal plane)respectively.

As shown in FIG. 5, second articulating surface 26 has a first radius ofcurvature 28 in the anterior-posterior direction that is preferablyconstant and preferably is between about 1 mm and about 30 mm, and morepreferably about 5 mm to about 10 mm, more preferably about 7.5 mm.Alternatively, the first radius of curvature 28 may vary along thesecond articulating surface 26 in the anterior-posterior direction.

Second curved articulating surface 26 has a first arc 22 in theanterior-posterior direction, measured from base point 1020 of edge 1021on the anterior side to base point 1030 on the posterior side of edge1021. Base points 1020 and 1030 are the end points of secondarticulating surface 26 in the anterior-posterior direction, and betweenwhich the surface is preferably continuously curved with no flatportions. First arc 22 of second articulating surface 26 preferably hasa length between about 3.0 mm to about 30 mm, more preferably about 10mm to about 15 mm, and more preferably about 13 mm to about 14 mm. Thelength of first arc 12 of first part 10 in the anterior-posteriordirection preferably is less than the length of first arc 22 of secondpart 20 in the anterior-posterior direction.

Second part 20 further has a first height 23 measured from base point1020, 1030, where the protrusion of convex articulating surface 26starts at edge 1021, to apex 1091 of second articulating surface 16.First height 23 preferably has a value between about 2 to about 5, morepreferably about 2 mm to about 3 mm, and more preferably about 2.5 mm.

As shown in FIG. 6, second articulating surface 26 has a second arc 42in the medial-lateral direction, measured from base point 1060 on themedial side of edge 1021 to base point 1070 on the lateral side of edge1021. Base points 1060 and 1070 are the end points on opposite ends ofsecond articulating surface 26 in the medial-lateral direction, andbetween which the surface is preferably continuously curved with no flatportions. Second arc 42 of second articulating surface 26 preferably hasa length between about 3.0 mm to about 30 mm, more preferably about 10mm to about 15 mm, and more preferably about 12 mm to about 14 mm.

As illustrated in FIG. 6, second articulating surface 26 preferably hasa second radius of curvature 48 in the medial-lateral direction. Thesecond radius of curvature 48 of second articulating surface 26preferably is between about 1 mm and about 30 mm, more preferablybetween about 5 mm to about 10 mm, and more preferably about 7.5 mm.Preferably, second radius of curvature 48 of second articulating surface26 is constant or substantially constant in the medial-lateraldirection. Alternatively, the second radius of curvature 28 may varyalong the second articulating surface 26 in the medial-lateraldirection.

Furthermore, second part 20 further has a second height 53 measured frombase point 1060, 1070, where the protrusion of convex articulatingsurface 26 starts, to apex 1091 of second articulating surface 26.Second height 53 preferably has a value between about 2 mm to about 5mm, more preferably about 2 mm to about 3 mm, and more preferably about2.5 mm

Referring to FIGS. 5-6, the value of first radius of curvature 28 ofsecond articulating surface 26 preferably is the same as the value ofsecond radius of curvature 48 of second articulating surface 26. Thelength of first arc 22 of second articulating surface 26 may be the samelength as second arc 42 of second articulating surface 26.Alternatively, the length of first arc 22 of second articulating surface26 may be greater than, or less than, the length of the second arc 42 ofsecond articulating surface 26. Preferably, the value of first height 23is equal to second height 53. Alternatively, the value of first height23 may be greater than, or less than, the value of second height 53.

Preferably the value of first height 23 of second part 20 is greaterthan the first depth 13 of first part 10. Additionally, the secondheight 53 preferably is greater than the second depth 43 of first part10. More preferably, the first height 23 is equal to the second height53, which is greater than the first depth 13, and the second depth 43.

Furthermore, referring to FIGS. 5-6, the first articulating surface 16does not match, correspond or nest with the second articulating surface26 so that the first articulating surface 16 does not contact the secondarticulating surface 26 over substantially either first articulatingsurface 16 or second articulating surface 26. More specifically, firstradius of curvature 18 of first articulating surface 16 preferably isconstant and preferably has the same value as first radius of curvature28 of second articulating surface 26 so that the first articulatingsurface 16 substantially matches and nests within the secondarticulating surface 26 in the anterior-posterior direction. However,the second radius of curvature 38 of first articulating surface 16varies along its length forming the shape of an ellipse and has adifferent value than the second radius of curvature 48 of secondarticulating surface 26, so that the first articulating surface 16preferably does not match the second articulating surface 26 in themedial-lateral direction. Preferably, second radius of curvature 38 ofthe first articulating surface 16 is greater than the second radius ofcurvature 48 of second articulating surface 26.

In the preferred embodiment, the convex, lower part 20 has a constantradius of curvature of about 7.5 mm in all directions and a height 23,53 of about 2.5 mm, and an arc length of about 11 mm in all directions.In the preferred embodiment, the concave, upper part 10 in theanterior-posterior direction has a constant radius of curvature of about7.5 mm, an arc length 12 of about 11 mm and a depth of about 1.8 mm,while in the lateral-medial direction has a radius 38 that varies alongthe length of arc 32, an arc length 32 of about 13 mm, and a depth ofabout 1.8 mm. Preferably the arc 32 is a partial ellipse with a theradius of curvature 38 in the medial-lateral direction that variesbetween about 1 mm and 100 mm.

The convex second articulating surface 26 may only contact the concavefirst articulating surface 16 over a limited area, less than the area ofeither the first articulating surface 16 or the second articulatingsurface 26. The value of this contact area may preferably comprise lessthan 50% of the area of first articulating surface 16 or secondarticulating surface 26. The value of this area more preferably maycomprise less than 25% of first or second articulating surfaces, andeven more preferably an area less than 10% of first or secondarticulating surfaces. Referring to FIGS. 5-6, first curved articulatingsurface 16 generally has a line or linear region of limited contact withsecond articulating surface 26 in the anterior-posterior direction.Preferably the first articulating surface 16 contacts the secondarticulating surface 26 generally along a line or region of limitedlength in the medial-lateral direction, but of greater length in theanterior-posterior direction. That is, the length of contact preferablyis greater in the anterior-posterior direction than the length ofcontact in the medial-lateral direction. The limited contact length inthe medial-lateral direction is shown in FIG. 6 where the convexarticulating surface 26 contacts the concave articulating surface 16along a band of limited length, and preferably at the apex 1090, 1091when the vertebrae are in their natural position with no forces actingon them from the musculoskeletal framework.

When a person moves from side to side, inducing lateral bending, themuscles cause vertebrae 101 and 201 from FIG. 3 to move about axis 1from FIG. 1. In turn, with intervertebral implant 100 inserted betweenadjacent vertebrae 101 and 201, the movement of the vertebrae causesfirst part 10 and second part 20, shown in FIGS. 3-6, to move relativeto one another. In the embodiment of FIGS. 5-6, where the convex secondarticulating surface 26 substantially matches the concave firstarticulating surface 16 in the anterior-posterior direction, and theconvex second articulating surface 26 does not match the concave firstarticulating surface 16 in the medial-lateral direction as a result ofits elliptical shape and varying radius of curvature, the first part 10will angularly rotate about the axis 1900 when undergoing flexion andextension.

Referring to FIG. 14, when the first radius of curvature 18 of firstarticulating surface 16 equals the first radius of curvature 38 ofsecond articulating surface 26, and the spine undergoes flexion, theupper or top first part 10 rotates about point 1900, which is the centerof the radius of curvature 28 formed on the second articulating surface26 in the second part 20. In the anterior-posterior direction, the firstradius of curvature 28 of the convex second part 20 may be locatedoutside of second part 20 (as shown in FIGS. 5 and 14) such that theconcave first part 10 rotates about the axis of rotation 1900. Dependingupon the geometry, the axis of rotation may also be located in theimplant, and in the second part 20. As further illustrated in FIG. 14,when first part 10 rotates about second part 20 toward the anterior,this results in an induced translation whereby first part 10 slides onthe second articulating surface 26 and shifts laterally toward theanterior with respect to the second part 20 from position 1101 toposition 1102.

Referring to FIG. 6, the shape of the first articulating surface 16 iselliptical or oval in the medial-lateral direction, the second radius ofcurvature 38 of first articulating surface 16 varies or changes over thesurface and is not equal to the second radius of curvature 48 of secondarticulating surface 26. Thus, when the spine undergoes lateral bendingand the first part 10 moves in the coronal plane (medial-lateraldirection) relative to the second part 20, the concave firstarticulating surface 16 rolls on the convex second articulating surface26. Referring to FIGS. 15A, 15B, and 15C, when second articulatingsurface 26 rolls, the point of contact 200 and the axis of rotation ofthe first part moves and follows the path of convex articulating surface26, thus rolling concave first articulating surface 16 over the convexsecond articulating surface 26. Referring to FIG. 15A, when no forcesare exerted onto first part 10 and second part 20, such that firstarticulating surface 16 and second articulating surface 26 are in astate of equilibrium, and the spine is in an upright position with noflexion, extension, lateral bending or axial rotation, first part 10 iscentered and aligned as shown in FIGS. 5-6 and 15A. As shown in FIG.15A, when first part 10 and second part 20 are in a state ofequilibrium, first part 10 is self-aligned with second part 20, meaningthat point of contact 200 in the medial-lateral direction corresponds toapex 1090, 1091, and, the apex 1090 of first part 10 coincides with theapex 1091 of second part 20. However, when forces are exerted onto firstpart 10 and second part 20 during lateral bending, first part 10 andsecond part 20 move in the medial-lateral direction as shown in FIG.15B, and contact point 200 and center of rotation of first part 10 movesto path point 404 shown in FIG. 15C. Thus, first part 10 rotatesrelative to second part 20, and rolls on and along the articulatingsurface of the second part.

As a result of the upper concave part 10 rolling along and on the lower,second articulating surface 26 of the second part 20 in themedial-lateral direction, such that the instantaneous axis of rotationmoves along the surface 26 in the medial-lateral direction, there is noinduced translation of the first part such that the upper concave part10 undergoes a larger angular motion before it contacts the uncinateprocess. Thus, the implant 100 permits approximately 9.2 degrees ofangular movement before the upper part contacts the uncinate process asopposed to the standard ProDisc-C sold by Synthes USA that generallypermits about 2 degrees of angular rotation in the coronal plane beforeimpingement with the uncinate process.

Another added benefit to the implant 100 is that the shape and geometryof the trough (concave surface) enables the bearing surface of theimplant to return to a state of minimum energy (apex to apex) afterlateral or rotational motion when the muscles no longer exert any forceson the spinal segment. That is the implant 100 will tend to move back toits natural position wherein the portion of the convex surface incontact with the concave surface is located at the apex of the curvateconcave articulating surface after the muscles no longer exert a forceon the spinal segment.

FIGS. 7-8 illustrate a second embodiment of an intervertebral implant100′. Intervertebral implant 100′ is similar to intervertebral implant100 except for the differences noted herein. Preferably the firstarticulating surface 16′ is concave, continuously curved with no flatspots, elliptically or oval shaped in both the anterior-posteriordirection and the medial-lateral direction, and has a radius ofcurvature that varies over the surface in all directions. Preferably,the second articulating surface 26′ is convex, continuously curved withno flat spots and is a partial sphere with a constant radius ofcurvature in all directions that is the same. Preferably the radius ofcurvature of the first articulating surface, although varying andchanging over its surface, is larger than the radius of curvature of thesecond articulating surface. The radius of curvature of the concavearticulating surface 16′ in the anterior-posterior and thelateral-medial directions may be between about 1 mm and 100 mm, andpreferably varies between 1 mm and 100 mm. The radius of curvature 18′and shape of the concave articulating surface in the anterior-posteriordirection may be the same as the radius of curvature 38′ and shape ofconcave articulating surface in the medial-lateral direction.

FIG. 7 illustrates a cross section of intervertebral implant 100′ in theanterior-posterior direction, while FIG. 8 illustrates a cross sectionof intervertebral implant 100′ in the medial-lateral direction. FIG. 7corresponds to FIG. 5, except that unlike the embodiment shown in FIG.5, the first radius of curvature 18′ of first articulating surface 16′in the embodiment of FIG. 7 is not equal to the first radius ofcurvature 28′ of second articulating surface 26′ in theanterior-posterior direction. Intervertebral implant 100′ in FIG. 8 issubstantially similar to intervertebral implant 100 in FIG. 6, whereinthe second radius of curvature 38′ of the first articulating surface 16′is not equal to the second radius of curvature 48′ of the secondarticulating surface 26′ in the medial-lateral direction.

Referring to FIGS. 7-8, preferably, the value of first radius ofcurvature 28′ of second articulating surface 26′ is equal to the valueof second radius of curvature 48′ of second articulating surface 26′.Alternatively, the value of the first radius of curvature 28′ of secondarticulating surface 26′ may be less than or greater than the value ofthe second radius of curvature 48′ of second articulating surface 26′.

The motion experienced by first part 10′ relative to second part 20′ inthe medial-lateral direction is similar to the motion experienced byintervertebral implant 100 in the medial-lateral direction. Concavefirst part 10′ will roll along the convex second part 20′ inmedial-lateral direction (when the spine undergoes lateral bending), andthe axis of rotation of the first part 10′ will be the point of contactbetween first part 10′ and second part 20′. Preferably, firstarticulating surface 16′ is concave and has, in both theanterior-posterior and medial-lateral directions, an elliptical or ovalshape and the radii of curvature 18′ and 38′ change along the length ofthe first articulating surface 16′. Preferably, the second articulatingsurface 26′ is convex and has constant radii of curvature 28′ and 48′ inboth the anterior-posterior and medial-lateral directions.

Preferably the radii of the first articulating surface 16′ do not matchthe radii of curvature of the second articulating surface 26′ in anydirection, including the anterior-posterior or medial-lateraldirections. Preferably the concave first articulating surface 16′ of theembodiment illustrated in FIGS. 7-8 contacts the convex secondarticulating surface 26′ over a limited area, and preferably over anarea of contact less than the area of contact of the embodimentillustrated in FIGS. 5-6. Preferably the area of contact of the firstarticulating surface 16′ with the second articulating surface 26 is lessthan 50%, preferably less than 25%, and more preferably less than 10% ofthe convex second articulating surface 26′. Preferably the firstarticulating surface 16′ contacts the second articulating surface 26′generally at a point or a region of limited contact that is generallycircular or partially spherical and a smaller area than the area ofsecond articulating surface 26′.

Furthermore, unlike intervertebral implant 100, intervertebral implant100′ has a projection or plateau 33′ in second part 20, as shown inFIGS. 7-8. Projection 33′ is measured from base point 1080′ to end point1030′. Projection 33′ may have a value between about 0 mm to about 5 mm.Projection 33′ provides the ability to produce different intervertebralimplants by varying the length of projection 33′. By providing secondparts 20′ of different heights 33′, different size implants can beassembled. For example, implants 100, 100′ can be assembled into desiredheights by supplying a kit with one or more first parts 10, 10′ and 20,20′ with different size projections 33, 33′ that may vary in 1 mmincrements. Thus, for example, a first part 10, 10′ may be supplied withprojections 33, 33′ of 0.5 mm, 1.5 mm, 2.5 mm, etc.

FIG. 11 is illustrates the path of motion experienced by vertebrae 101and 201 of FIG. 1 when vertebrae 101 and 201 are located in the lumbarregion of the spine. Curved line 6″ represents the motion that should beexperienced by healthy lumbar vertebrae relative to one another. Unlikethe cervical region, in the lumber region the path 6′ of the lumbervertebrae in the axial plane is along the circumference of a circle, asopposed to an ellipse. More specifically, curved path 6″ represents thepath of the instantaneous axis of rotation of the upper lumbar vertebrae101″ along and over the lower lumbar vertebrae 102″. Bean shape 16″represents a widened surface area projected along path 6″ against andwithin the lumbar vertebrae.

FIG. 12 illustrates where convex articulating surface 26 of second part20 would be located in the lumbar region of the spine to contain theinstantaneous axis of rotation of the upper adjacent lumbar vertebrae.More specifically, range lines 300 and 302 represent the maximummovement that the instantaneous axis of rotation would experience whenprojected on the lower lumbar vertebra. Based on this maximum range ofmovement of the axis of rotation, the shape of first articulatingsurface 16″ for lumbar intervertebral implant prosthesis is created asshown in FIG. 13.

FIG. 13 illustrates a bottom view of the top part 10″ of anintervertebral implant 100″ designed to be implanted in the lumbarregion of the spine. Intervertebral implant 100″ contains a first part10″ which contains a first curved concave articulating surface 16″ asshown in FIG. 13. First articulating surface 16″ has a bean like shapeto accommodate the motion exerted on intervertebral implant 100″ by thevertebrae located in the spine's lumbar region. In FIG. 13 the secondconvex articulating surface 26″ of second part 20″ is projected on theconcave first articulating surface 16″ as the circular section. As thelumbar vertebrae move, first curved concave articulating surface 16″moves on the convex second articulating surface 26″. As can be seen inFIGS. 9 and 13, the first articulating surface 16″ of intervertebralimplant 100″ of FIG. 13 has a much larger area for containing the secondarticulating surface 16″ in the lumbar region than the area of firstarticulating surface 16 in FIG. 9. Therefore, due to this extended beanlike shape of first articulating surface 16″, intervertebral implant100″ preferably accommodates an axial rotation of up to ±10 degrees inthe lumbar region, as opposed to ±7 degrees in the cervical region ofthe spine.

The shape of the first articulating surface in the medial-lateraldirection may be shaped as a circle or sphere, or alternatively as anellipse or oval with a radius of curvature that varies. The radius ofcurvature of the first, preferably concave, articulating surface ispreferably between about 20 mm and about 100 mm. The concave surface 16″will be longer than the concave surface 16 or 16′ and will also appearmore curved due to its distance to the axis of rotation. The radius ofcurvature of the first, preferably concave, articulating surface in thelateral-medial direction is greater than the radius of curvature in theanterior-posterior direction, and preferably is greater than the radiusof curvature of the second, preferably convex, articulating surface 26″.The second articulating surface 26″ preferably is spherical in shape andhas a constant radius of curvature in all directions. The radius ofcurvature of the second, preferably convex, articulating surface in thelateral-medial direction is preferably different than the radius ofcurvature of the first, preferably concave, articulating surface in thelateral-medial direction so that the first articulating surface does notmatch, correspond or nest with the second articulating surface in thelateral-medial direction. Preferably, the concave articulating surfacewill roll on and along the convex articulating surface so that theinstantaneous axis of rotation of the concave part about the convex partmoves along the surface of the convex part.

It should be understood that the application is not limited to theprecise arrangement, structures, features, embodiments, aspects, andinstrumentalities shown, and that the arrangements, structures, featuresand instrumentalities shown may be used singularly or in combinationwith other arrangements, structures, features, aspects andinstrumentalities. For example, the intervertebral implant 100illustrated in the embodiment of FIGS. 5-6 may contain a projection 33to increase the height of intervertebral implant 100. Additionally, forexample, while the concave articulating surface has been shown anddescribed as associated with the superior vertebrae and the convexarticulating surface has been shown and described as associated with theinferior vertebrae, one of skill in the art could readily appreciatethat the concave and convex surfaces can be switched and/or the convexsurface can be elliptically parabollically shaped in one or moredirections while the concave surface is partially spherical.Additionally, while the implant has been shown and described as having afirst part and a second part, with each of first part and second partbeing a single monolithic piece, each of first and second part may bemultiple pieces assembled together as well known in the art.Accordingly, first part or second part, or both, may comprise one pieceto contact, engage, and secure to the vertebrae and another piececomprising the articulating or bearing surface, the two pieces beingcoupled together to form the first or second parts.

As will be appreciated by those skilled in the art, any or all of thecomponents described herein may be provided in sets or kits so that thesurgeon may select various combinations of components to form an implantand create a disc replacement system which is configured specificallyfor the particular needs/anatomy of a patient. It should be noted thatone or more of each component may be provided in a kit or set. In somekits or sets, the same component or part may be provided in differentshapes and/or sizes. The surgeon or staff may mix and match the firstand second parts to create the implant before or during the procedure.

While the foregoing description and drawings represent the preferredembodiments of the present invention, it will be understood that variousadditions, modifications, combinations and/or substitutions may be madetherein without departing from the spirit and scope of the presentinvention as defined in the accompanying claims. In particular, it willbe clear to those skilled in the art that the present invention may beembodied in other specific forms, structures, arrangements, proportions,and with other elements, materials, and components, without departingfrom the spirit or essential characteristics thereof. One skilled in theart will appreciate that the invention may be used with manymodifications of structure, arrangement, proportions, materials, andcomponents and otherwise, used in the practice of the invention, whichare particularly adapted to specific environments and operativerequirements without departing from the principles of the presentinvention. In addition, features described herein may be used singularlyor in combination with other features. The presently disclosedembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims, and not limited to the foregoingdescription.

What is claimed:
 1. An intervertebral implant for insertion into anintervertebral space between a first vertebra and a second vertebra, theimplant comprising: a first part including a first outer surfaceconfigured to contact an end plate of the first vertebra, and a convexinner surface spaced from the first outer surface along a firstdirection, the convex inner surface having a first radius of curvaturealong a first plane defined along the first direction and a seconddirection, perpendicular to the first direction, and a second radius ofcurvature along a second plane defined along the first direction and athird direction, perpendicular to both the first and second directions;a second part including a second outer surface configured to contact anend plate of the second vertebra, and a concave inner surface that isspaced from the second outer surface along a direction opposite thefirst direction and that receives the convex inner surface so that thefirst part is moveable with respect to the second part, the concaveinner surface having a first radius of curvature along a third plane,parallel to the first plane, and a second radius of curvature along afourth plane, parallel to the second plane, wherein the first radius ofcurvature of the convex inner surface is different than the first radiusof curvature of the concave inner surface, and the second radius ofcurvature of the convex inner surface is different than the secondradius of curvature of the concave inner surface, and wherein at leastone of the concave inner surface and the convex inner surface iscontinuously curved from a first end point to a second point, and thefirst radius of curvature of the one of the concave inner surface andthe convex inner surface varies between the first and second end points.2. The intervertebral implant of claim 1, wherein the second radius ofcurvature of the one of the concave inner surface and the convex innersurface varies.
 3. The intervertebral implant of claim 1, wherein atleast one of the first and second radii of curvature of the other one ofthe concave inner surface and the convex inner surface is constant. 4.The intervertebral implant of claim 1, wherein the first radius ofcurvature of the concave inner surface is greater than the first radiusof curvature of the convex inner surface.
 5. The intervertebral implantof claim 4, wherein the second radius of curvature of the concave innersurface is greater than the second radius of curvature of the convexinner surface.
 6. The intervertebral implant of claim 1, wherein thefirst part is a lower part and the second part is an upper part.
 7. Theintervertebral implant of claim 1, wherein the concave inner surface isin the shape of a partial oblate ellipsoid.
 8. The intervertebralimplant of claim 7, wherein the convex inner surface is in the shape ofa partial sphere.
 9. The intervertebral implant of claim 1, wherein theconcave inner surface only contacts the convex inner surface on an areacomprising less than 50% of the area of the convex inner surface. 10.The intervertebral implant of claim 9, wherein the concave inner surfaceonly contacts less than 25% of the convex inner surface.
 11. Theintervertebral implant of claim 9, wherein the concave inner surfaceonly contacts less than 10% of the convex inner surface.
 12. Theintervertebral implant of claim 1, wherein the convex inner surfaceforms approximately point contact with the concave inner surface. 13.The intervertebral implant of claim 1, wherein the concave and convexinner surfaces are continuously curved with no flat portions.
 14. Theintervertebral implant of claim 1, wherein the second part includes aninner surface that is opposite the second outer surface and faces thefirst part, and the concave inner surface is (i) continuously curved inthe first plane from the first end point on the inner surface to thesecond end point on the inner surface and (ii) continuously curved inthe second plane from a third end point on the inner surface and to afourth end point on the inner surface.
 15. An intervertebral implant forinsertion into an intervertebral space between a first vertebra and asecond vertebra, the implant comprising: a first part including a firstouter surface configured to contact an end plate of the first vertebra,and a convex inner surface spaced from the first outer surface along afirst direction, the convex inner surface having a first radius ofcurvature along a first plane defined along the first direction and asecond direction, perpendicular to the first direction, and a secondradius of curvature along a second plane defined along the firstdirection and a third direction, perpendicular to both the first andsecond directions; a second part including a second outer surfaceconfigured to contact an end plate of the second vertebra, and a concaveinner surface that is spaced from the second outer surface along adirection opposite the first direction and that receives the convexinner surface so that the first part is moveable with respect to thesecond part, the concave inner surface having a first radius ofcurvature along a third plane, parallel to the first plane, and a secondradius of curvature along a fourth plane, parallel to the second plane,wherein the first radius of curvature of the convex inner surface isdifferent than the first radius of curvature of the concave innersurface, the second radius of curvature of the convex inner surface isdifferent than the second radius of curvature of the concave innersurface, and the first radius of curvature of one of the concave innersurface and the convex inner surface varies, wherein: the convex innersurface is (i) continuously curved in the first plane from a first endpoint to a second end point and (ii) continuously curved in the secondplane from a third end point and a fourth end point; and the first partincludes a first inner surface that is opposite the first outer surfaceand faces the second part, and includes a projection that spaces thefirst to fourth end points away from the first inner surface.
 16. Theintervertebral implant of claim 15, wherein one of the concave innersurface and the convex inner surface is continuously curved from a firstend point to a second point, and the first radius of curvature of theone of the concave inner surface and the convex inner surface variesbetween the first and second end points.
 17. The intervertebral implantof claim 15, wherein the concave and convex inner surfaces arecontinuously curved with no flat portions.